fix: avoid kanban session_id index migration crash

[liowald]

Summary

• move idx_tasks_session_id creation out of the static schema script
• create the index after the additive session_id migration has run
• preserves the index for fresh databases and legacy migrated databases

Test Plan

• python -m pytest tests/hermes_cli/test_kanban_db.py tests/hermes_cli/test_kanban_db_init.py tests/hermes_cli/test_kanban_boards.py -q -o 'addopts='

[alt-glitch]

Duplicate of #28461 — same kanban session_id index migration fix. Already resolved by the comprehensive salvage in #28781 (merged). See also #28776, #28562, #28597, #28693 (all duplicates).

[teknium1]

Closing as superseded by #28781.

Triage notes (high confidence):
Same idx_tasks_session_id-out-of-SCHEMA_SQL fix was merged via PR #28781 (kanban_db.py:1369 on main); PR is CONFLICTING and redundant.

Thanks for the contribution — the underlying problem this PR addresses has been resolved by the linked PR on current main. If you believe this was closed in error, please comment and we'll reopen.

(Bulk-closed during a CLI PR triage sweep.)